1. Field of the Invention
The present invention relates to a method and apparatus for assisting a heart to pump blood by applying uniform pressure to at least a portion of the ventricular surface of the heart thereby causing the respective ventricle portion of the heart to be substantially uniformly deformed.
2. Discussion of the Related Art
Numerous attempts have been made to assist the heart by applying external pressure directly to the heart. One such example is direct manual compression of the heart by a person's hand during open chest cardiopulmonary resuscitation. Often, however, it may be advantageous for the patient if cardiac or circulatory support is performed by compressing the heart for extended periods of time, such as hours, days or weeks, and it is quite difficult for medical personnel to apply a rhythmic pulsating pressure for an extended period of time. Further, it is impossible to apply by hand a uniform compressing force to a majority portion of the exterior ventricle surface of the chamber of the heart.
To overcome this problem, mechanical devices have been developed to apply external pressure directly to the heart. These devices utilize an inflatable liner that surrounds the heart. For example, U.S. Pat. Nos. 3,455,298 and 5,119,804, both to Anstadt, disclose a cup that is provided with an elastomeric liner. The heart is held in place within the liner, which is cyclically inflated and deflated to apply external pressure to the heart. While this device provided an improvement in hemodynamics, it has several disadvantages.
One disadvantage is that only a fraction of the external fluid pressure that is applied in the cup inlet to displace the liner, which in turn displaces the heart wall, is transmitted to supplement the internal pressure of the heart and is used to pump blood. As the liner is inflated and stretched, a transmural pressure is created in the liner. The transmural pressure in the liner is the difference in pressure that is applied to both sides of the liner. In other words, the transmural pressure is the pressure within the liner that is generated by the elastic wall tensions of the liner. Referring now to FIG. 11, and as described in Augmentation of Pressure In A Vessel Indenting the Surface of the Lung, 1987, by Joshua E. Tsitlik, et al. the transmural pressure (P.sub.tm) for a stretched liner is: EQU P.sub.tm =P.sub.in -P.sub.out =T.sub.1 /R.sub.1 +T.sub.2 /R.sub.2
Where the radii R.sub.1 and R.sub.2 are the maximum and the minimum radii of the membrane curvature, respectively, i.e., the principal radii of curvature. The vectors T.sub.1 and T.sub.2 are the elastic wall tensions (the force per unit length) acting along the edges of the surface element.
In practice, as the liner is inflated, because of its axial length limitation, it stretches and bulges radially inwardly. Thus, the transmural pressure of the liner is directed in the radially outward direction. Therefore, the pressure applied to the heart is less than the pressure applied to the liner. In addition, due to the bulging of the liner, the heart is deformed into an hour-glass shape. In other words, the outer central portion of the surfaces of the ventricles of the heart is deformed inwardly from its normally convex shape into a concave shape (i.e., the heart is indented). The transmural pressure of the indented portion of the heart wall is directed in the radially outward direction and, thus, is subtracted from the fluid pressure that is supplied within the liner to the outer surface of the heart. Thus, this transmural pressure is also subtracted from the fluid pressure that is applied within the liner. In other words, the heart wall itself is fighting against the externally applied force. Thus, the externally applied force in the Anstadt type device does not cooperate with the heart's own natural compressive forces during the systolic phase. It actually fights against the heart's natural motion even when the pressure is applied in synchrony with the natural systolic phase of the heart.
Consequently, the fluid pressure applied within the liner must overcome the transmural pressure created in the liner and in the heart wall. Therefore, a relatively high pressure must be applied within the liner (e.g., 150-200 mm Hg) to achieve any assistance in circulation support.
Thus, the liner of the Anstadt patents applies pressure to the heart in a non-uniform manner. Such liners are made from a silicone rubber elastomer, which, when inflated, are inherently distended and assume the shape illustrated in FIG. 9 of the '804 patent and FIG. 3 of the '298 patent. The liner of the Anstadt patents causes the heart to indent in its center portion, while, undesirably, allowing the heart ventricles to expand at their upper and lower portions. Thus, the devices according to the Anstadt patents inefficiently assist in pumping blood to and from the heart. As a result, substantial force needs to be applied to the pressure side of the liner (i.e., P.sub.in) to cause displacement of the blood existing in the ventricle. A considerable portion of the force that is applied to the pressure side of the liner is wasted because a transmural pressure is created in the liner and the heart wall.
Another disadvantage of the prior art stems from the same feature. The increased force or pressure applied to the central portion of the ventricles' outer surface causes the heart to deform into an unnatural shape and eventually causes trauma (e.g., bruises) to the heart, especially if the prior art device is operated for a relatively extended period of time.
The ventricle portion of the heart normally has an outer convex shape during both the systolic and diastolic phases. The present inventors have surprisingly discovered that if a predetermined portion of the ventricle portion of the heart is assisted at a given externally and uniformly applied force, during the systolic phase, into its natural, albeit somewhat smaller in volume, convex shape, a great increase in the amount of fluid being pumped by the heart can be achieved compared to the same force applied according to the prior art methods. This is due, as the inventors discovered, to the fact that in the present invention the pressure applied to the heart surface and the pressure generated by contracting the heart wall add up to produce higher pressures in the ventricles without additional oxygen consumption by the ventricular wall. Therefore, by the non-distorting compression of the heart, the present invention assists the heart in generating higher blood pressure and blood flow while utilizing low and safe compression pressures.
Thus, it is an object of the present invention to provide an apparatus and method for efficiently assisting the mechanical compression of the heart, especially during the systolic phase. It is another object to provide such an apparatus and method without unduly deforming the natural shape of the heart during the mechanical compression of the heart.
It is a further object of the present invention to provide a device and method for assisting heart function, which applies substantially uniform fluid pressure against the exterior surface of at least a portion of the ventricular portion of the heart during the systolic phase.
The devices disclosed by the Anstadt patents each apply a vacuum pressure to a lower portion of the cup to prevent the heart from being ejected from the cup during the systolic compressing phase. In the '298 patent, Anstadt discloses that the vacuum communicates through opening 16 at the bottom of the liner and carries vacuum into space 6. However, in practice, only a relatively small portion of the apical portion of the heart actually plugs opening 16 and, therefore, the vacuum holding force is applied only to the portion of the heart proximate to opening 16. The application of a relatively large vacuum to such a small surface of the heart, especially in view of the large external force that needs to be applied to perform the holding function, causes further trauma to the heart. Accordingly, it is a further object of the present invention to provide an apparatus that applies a vacuum holding force over a greater relatively flat side surface area of the heart. It is still a further object of the present invention to provide a device and method that applies a vacuum holding force in such a manner so as to cause considerably less trauma to the heart.